In an inductively coupled plasma (ICP) emission spectrometer, a plasma-generating gas (e.g. argon gas) is ionized by an electromagnetic field created by supplying radio-frequency power to an induction coil. While the obtained plasma is maintained by the electromagnetic field, a sample atom is introduced into the plasma. The sample atom is excited by the plasma, and when the excited atom returns to a lower energy level, it emits light whose wavelength is specific to the atom. By performing a spectrometry of the emitted light, a qualitative and quantitative determination of the sample is performed.
In an ICP emission spectrometer, in order to supply radio-frequency power to the plasma, a configuration is widely used in which an LC resonation circuit formed by an induction coil and a capacitor is driven by a radio-frequency power unit which can supply, for example, radio-frequency power of several hundred watts to several kilowatts at a frequency of 27 MHz. In such a configuration, to enable the radio-frequency power unit to efficiently operate, the load impedance seen from the radio-frequency power unit should preferably be constant, and furthermore, the impedance should be matched with the optimum load impedance of the power unit.
However, when plasma is generated by passing a radio-frequency current through the induction coil, the impedance of the induction coil changes due to the effect of the induction current caused by the movement of charged particles in the plasma. The impedance of the induction coil also changes with a change in the state of the plasma, which occurs depending on the state of the plasma-generating gas or that of the sample to be analyzed, the amount of power supplied to the plasma, and other factors. Such a change in the impedance of the induction coil leads to a change in the load impedance seen from the radio-frequency power unit, causing the impedance matching to deviate from the optimum state.
To overcome this problem, a self-oscillating circuit has been commonly used in this type of radio-frequency power unit, in which the LC resonance circuit which includes the induction coil is driven by a switching circuit (such as a half-bridge or full-bridge circuit including a semiconductor switching element consisting of a MOSFET or the like), with the positive feedback of a signal from the LC resonation circuit to a control terminal (which is the gate terminal in the case of a MOSFET) of the semiconductor switching element via a transformer or similar component.
FIG. 5 is a schematic configuration diagram of a radio-frequency power unit using a self-oscillation, which is described in Patent Literature 1. In this circuit, four MOSFETs 141, 142, 143 and 144 form a full-bridge circuit. Electric power is supplied from this full-bridge circuit to an LC resonance circuit 31 including an induction coil 32 and a capacitor 33 via inductors 301, 302, 303 and 304. The thereby generated flow of current in the LC resonance circuit 31 pass through the primary winding 34 of a transformer (inductive coupler) and fed back through a feedback path (not shown) to the gate electrode of each of the MOSFETs 141-144. In such a self-oscillating circuit, when the impedance of the induction coil 32 changes depending on a change in the state of the plasma, the resonant frequency of the LC resonance circuit 31 automatically changes. This produces an advantageous result whereby the load impedance seen from the full-bridge circuit is constantly maintained at optimum levels, allowing the oscillation to continue with high efficiency without requiring any special control or command from outside sources.